Flashcards in Chapter 3- Meiosis Deck (24)
Why is meiosis known as reduction division?
Because it involved halving the chromosome number
What does the halving of chromosomes allow?
Allows a sexual life cycle with fusion of gametes
What is fertilisation?
Is the Union of sex cells, or gametes, usually from different parents
What happens just before meiosis?
DNA is replicated so that all chromosomes consist of two sister chromatids
What do the early stages of meiosis involve?
Pairing of homologous chromosomes and crossing over followed by condensation
First couple of stages of meiosis
1) homologous chromosomes pair up with each other
2) because DNA replication has already occurred, each chromosome consists of two chromatids and so there are four DNA molecules associated in each pair of homologous chromosomes
3) a pair of homologous chromosomes is bivalent and the pairing process is sometimes called synapsids
What happens after synapsis?
Process called crossing over takes place - outcome is that a junction is created where one chromatid in each homologous breaks up and rejoins win the other chromatid
What is crossing over?
Occurs random positions anywhere along the chromosomes
The orientation of homologous chromosomes is what?
Prior to separation is random which means meiosis creates genetically diverse haploid cells
What is disjunction?
Two chromosomes in each bivalent are held together by chiasmata but these slide to the end of the chromosomes and then the chromosomes can separate - one chromosome from each bivalent moves to one of the poles and the other chromosomes to the other pole
What does disjunction do?
Half the chromosomes- which means the first division is the reduction devision
What are two methods of obtaining cells for karyotype?
Chronic villus sampling and amniocentesis
What is aminocentasis?
Involves the passing of a needle through the mothers abdomen- the needle is used to withdraw a sample of amniotic fluid containing feral cells from the amniotic sac
What is chorionic villus?
A sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta developed
What is the risk of miscarriage from the 2 ways of obtaining cells a?
Aminocentasis = 1%
Chronic villus = 2%
Explain the first division of meiosis
Prophase I : 1) homologous chromosomes pair (snyapsis)
2) crossing over occurs
Metaphase I : 1) microtubules move homologous pairs to equator of cell
2) orientation of paternal and maternal is random
Anaphase I: homologous pairs are separated one chromosome of each pair moves to each pole
Telophase I: 1) chromosome uncoil
2) reduction of chromosome number from diploid to haploid completed
3) cytokinesis occurs
Explain the second devision of meiosis
Prophase II: chromosome which still consists of two chromatids, condense and become visible
Anaphase II: centromeres separate and chromatids are moved to opposite poles
Telophase II: 1) chromatids reach opposite poles
2) nuclear envelope forms
3) cytokinesis occurs
What does crossing over and random orientation promote ?
How does random orientation of bivalent sallow genetic variation?
For a haploid number of n, the number of possible combinations is 2^n
why does th fusion of gametes from different parents promote genetic variation
1) it is the start of the lift of a new individual
2) it allows alleles from two different individuals to be combined in one new individual
3) combination of alleles is unlikely to ever have existed before
4) fusion of gametes therefore promotes genetic variation in a species
5) genetic variation is essential for evolution
What is non - disjunction the cause of?
Down syndrome and other chromosome abnormalities
Why does non-disjunction happen?
When homologous fail to separate at anaphase- results will be either that a gamete had an extra chromosome or one less
What do studies of age and non disjunction show?
That age influences chances of non- disjunction